Transaction and Optimistic Concurrency Control

In this chapter, you will implement all interfaces of Transaction. Your implementation will maintain a private workspace for modifications inside a transaction, and commit them in batch, so that all modifications within the transaction will only be visible to the transaction itself until commit. We only check for conflicts (i.e., serializable conflicts) when commit, and this is optimistic concurrency control.

To run test cases,

cargo x copy-test --week 3 --day 5
cargo x scheck

Task 1: Local Workspace + Put and Delete

In this task, you will need to modify:

src/txn.rs

You can now implement put and delete by inserting the corresponding key/value to the local_storage, which is a skiplist memtable without key timestamp. Note that for deletes, you will still need to implement it as inserting an empty value, instead of removing a value from the skiplist.

Task 2: Get and Scan

In this task, you will need to modify:

src/txn.rs

For get, you should first probe the local storage. If a value is found, return the value or None depending on whether it is a deletion marker. For scan, you will need to implement a TxnLocalIterator for the skiplist as in chapter 1.1 when you implement the iterator for a memtable without key timestamp. You will need to store a TwoMergeIterator<TxnLocalIterator, FusedIterator<LsmIterator>> in the TxnIterator. And, lastly, given that the TwoMergeIterator will retain the deletion markers in the child iterators, you will need to modify your TxnIterator implementation to correctly handle deletions.

Task 3: Commit

In this task, you will need to modify:

src/txn.rs

We assume that a transaction will only be used on a single thread. Once your transaction enters the commit phase, you should set self.committed to true, so that users cannot do any other operations on the transaction. You put, delete, scan, and get implementation should error if the transaction is already committed.

Your commit implementation should simply collect all key-value pairs from the local storage and submit a write batch to the storage engine.

Test Your Understanding

  • With all the things we have implemented up to this point, does the system satisfy snapshot isolation? If not, what else do we need to do to support snapshot isolation? (Note: snapshot isolation is different from serializable snapshot isolation we will talk about in the next chapter)
  • What if the user wants to batch import data (i.e., 1TB?) If they use the transaction API to do that, will you give them some advice? Is there any opportunity to optimize for this case?
  • What is optimistic concurrency control? What would the system be like if we implement pessimistic concurrency control instead in Mini-LSM?

Bonus Tasks

  • Spill to Disk. If the private workspace of a transaction gets too large, you may flush some of the data to the disk.

Your feedback is greatly appreciated. Welcome to join our Discord Community.
Found an issue? Create an issue / pull request on github.com/skyzh/mini-lsm.
Copyright © 2022 - 2024 Alex Chi Z. All Rights Reserved.